Screw pumps (also referred to as progressive cavity pumps) are widely used in the oil industry to pump oil from wells. Generally, the operations of the screw pump are controlled by a pump control system including an electric motor and a motor drive. The screw pump comprises a pump rod having a stator and a rotor. The pump rod of the screw pump is physically located deep within the oil well for pumping the oil to the surface. The geometry of the assembly of the stator and the rotor constitutes two or more series of spiral and separate cavities. The electric motor is configured to rotate the rotor of the screw pump. When the rotor rotates inside the stator, the cavities move spirally from one end of the stator to the other and a positive displacement pumping action is created so as to lift the oil to the surface.
When the screw pump is in normal pumping operation, the screw pump driven by the electric motor can provide energy to wind up the pump rod so as to lift the oil to the surface. One of the more significant problems encountered with the pumping operation of the screw pump is the “backspin” that may occur in the event of a momentary electrical power interruption. When the electrical power is lost, the screw pump loses the ability to control the energy stored in the pump rod due to the oil load thereon. However, since a very large amount of stored energy still exists in the pump rod of the screw pump, the action of the screw pump is similar to a wound coil spring. The stored energy is released through backspin of the pump rod of the screw pump, causing the rotor of the screw pump to rotate in the opposite direction. The pump rod of the screw pump will spin in the reverse direction until all the oil has fallen back down the production tube and the oil level in the production tube and the well are equal due to gravity. Depending on the pump applications, the backspin time of the screw pump can last for several hours. Under this circumstance, the pumping operation of the screw pump cannot be restarted immediately when the electrical power is provided again. The time period spent during backspin and the time period of waiting for the oil to get back to the ground level (after restarting) will lose productivity. Consequently, electrical power interruption can cause a significant loss in screw pump productivity.
In addition, when the motor drive is shut down by a user, the pump operation of the screw pump is decelerated and stopped by a barking device according to a scheduled shut down procedure. When the motor drive stops providing the control voltage to the electric motor and the electric motor is shut off, the stored energy is released through backspin of the pump rod of the screw pump at very high backspin speeds. Since the driving mechanism of the electric motor is often directly connected to the rotor of the screw pump, the electric motor will be subjected to backspin. Uncontrolled backspin can severely damage the drive mechanism and other production equipment. In some instances, the backspin may result in equipment destruction. In addition, if such destruction occurs at the ground level of the well, there exists the possibility of personal injury and environmental contamination.
Therefore, there is a need of providing a control method and a control system for controlling the operation of a screw pump to eliminate the effects of backspin when electrical power is lost so as to obviate the drawbacks encountered from the prior arts.